The present invention relates to a stretch-formed sheet of ultra-high molecular weight polyolefin having excellent transparency and mechanical properties, and to a production method thereof; in particular, it relates to a stretch-formed sheet of ultra-high molecular weight polyolefin, which can be industrially produced, is relatively thick, and has excellent transparency and mechanical properties, and to a production method thereof.
Compared to high molecular weight polyolefin used in general, formed articles of ultra-high molecular weight polyolefin represented by ultra-high molecular weight polyethylene, or the like, have excellent mechanical properties, such as, impact resistance, abrasion resistance, low coefficient of friction and tensile strength, and resistance to chemicals. Taking advantage of such characteristics, formed articles of ultra-high molecular weight polyolefin are used in mechanical parts such as gears, pulleys or sprockets, in various linings of guide rails or hopper tank lining materials, and the like.
Meanwhile, since molten viscosity is extremely high and fluidity is poor for ultra-high molecular weight polyolefins compared to polyethylene used in general, which makes forming by extrusion molding and injection molding extremely difficult, there is the disadvantage that forming processes are difficult. Therefore, compression molding, or the like, has been adopted in general as a forming method for ultra-high molecular weight polyethylene, and as part of others, a method has been adopted, called ram extrusion molding, in which molding is under the condition of extremely low molding speed, in order to obtain rod-shaped formed products.
In addition, ski sole or the like exists as one application of ultra-high molecular weight polyethylene, and sheet material applications requiring this type of high mechanical properties are thought to be ones that will take advantage of the characteristics that an ultra-high molecular weight polyethylene has.
In recent years, in this type of application, sheets having excellent transparency are sought, with the purpose of increasing designability of letters, drawings and the like drawn on the bottom of a ski sole, for instance. However, formed article obtained from crystalline resin of polyolefin represented by polyethylene, in general, is in a poorly transparent cloudy state, the formed article per se being white. That is to say, the current situation is that nothing fulfilling sufficiently the performance regarding transparency has been obtained yet.
Many films, sheets, pipes, and the like, using low density polyethylene have been reported so far in prior art as formed articles of polyethylene having excellent transparency, achieving high transparency. However, sufficient effects on the point of mechanical properties such as abrasion resistance and impact resistance cannot be expected from those using low density polyethylene, as the molecular weight is lower compared to ultra-high molecular weight polyethylene.
Meanwhile, as ultra-high molecular weight polyethylene having low density, in general, those in which ethylene and α-olefin have been copolymerized are known, and a low density ultra-high molecular weight polyethylene is disclosed, for instance, in Patent Reference 1. However, the low density ultra-high molecular weight polyethylene described in Patent Reference 1, has a comparatively high proportion of degree of crystallinity with respect to density, and is not sufficient in terms of transparency.
In addition, even if they have the same transparency, formed articles of ultra-high molecular weight polyethylene having higher density and degree of crystallinity are desired. That is to say, if the density is decreased to lower the degree of crystallinity in order to increase transparency, mechanical properties of this formed article, such as, tensile yield strength, modulus of elongation and flexural strength decrease, which is not desirable.
Here, in order to improve transparency, it is possible to copolymerize a greater number of comonomers such as α-olefin to decrease the density further. However, from such reasons as the molecular weight decreases while the copolymerization reaction between ethylene and α-olefin progresses and the density decreases, obtaining an ultra-high molecular weight polyethylene having a high molecular weight and a low density is extremely difficult by such improvements. Moreover, copolymerizing a greater number of comonomers such as α-olefin is not only uneconomical, mechanical properties such as stiffness decrease remarkably along with the decrease in the density. Therefore, obtaining a sufficient performance comprising high transparency and mechanical properties is not possible by such methods.
Meanwhile, a method for transparentization is known, whereby the degree of crystallinity is reduced by rapidly cooling after melting. For instance, disclosed in Patent Reference 2 is a production method for a sheet in which the density in a coating material for a ski using an ultra-high molecular weight polyethylene has been lowered by cooling rapidly after re-melting the formed article. However, the strip material containing the ultra-high molecular weight polyethylene described in Patent Reference 2 had originally a high density and an insufficient transparency in the formed article. In addition, in such a method there are technical limitations to lower the density by further cooling rapidly to improve transparency. Moreover, as this method requires cumbersome steps such as carrying out heat treatment again in a subsequent step, then cooling rapidly, it is not economical.
In addition, in prior art, methods such as rolling and drawing are adopted as means for stretching polyethylene material in general, and, disclosed in Patent References 3 to 5, for instance, are methods in which rolling, drawing, and the like, are carried out with the purpose of reducing slidability and abrasion, surface smoothness and low coefficient of friction. In addition, disclosed in Patent Reference 6 is a production method for preparing a high density polyethylene sheet having improved transparency and mechanical properties by heat compressing and then re-drawing high density polyethylene.
Meanwhile, techniques for stretching polyethylene materials having a low molecular weight to a high degree are already in practical use in the technical domains of films and strings, and there is transparent polyethylene as a representative example thereof. However, regarding mechanical properties and the like, nothing has been obtained that would satisfy performance as sufficiently as formed articles of ultra-high molecular weight polyethylene, owing to the low molecular weight. For instance, disclosed in Patent Reference 7 is a production method for preparing a highly strong sheet-shaped product having excellent transparency by rolling. However, due to the sheet-shaped product of Patent Reference 7 having a low molecular weight, mechanical properties such as abrasion resistance and impact resistance are not sufficient.
Meanwhile, disclosed in Patent Reference 8 and 9 are production methods for preparing a highly strong polyethylene ultra-thin film having excellent transparency by biaxially drawing a gel-state sheet with a high drawing factor. However, with the methods disclosed in Patent Reference 8 and 9, cumbersome steps are required in order to prepare the gel-state sheet, such as solvent removing process and heat-pressurizing process to transparentize the multipored opaque draw-formed product, which is not economical. Moreover, as they are for preparing ultra-thin film on the order of several μm to 10 and several μm by biaxially drawing a gel-state sheet with a high drawing factor, obtaining a sheet having excellent transparency with a thickness of on the order of several hundreds of μm or greater is not possible.
In addition, disclosed in Patent Reference 10 as methods for drawing an ultra-high molecular weight polyethylene to a high degree are methods in which biaxial drawing is carried out in the presence of a plasticizer (flow improver). However, it is supposed that with this method, thick product having excellent transparency cannot be obtained, due to holes appearing after plasticizer extraction and the product becoming multipored, giving rise to light scattering caused by this porosity.    Patent Reference 1: National Publication of Translated version No. H5-86803    Patent Reference 2: Japanese Patent Publication No. 3491835    Patent Reference 3: Japanese Patent Application Laid-open No. 2006-001098    Patent Reference 4: Japanese Patent Application Laid-open No. H11-147246    Patent Reference 5: Japanese Patent Application Laid-open No. H6-91747    Patent Reference 6: Japanese Patent Application Laid-open No. H10-323892    Patent Reference 7: Japanese Patent Application Laid-open No. H7-164461    Patent Reference 8: Japanese Patent Application Laid-open No. S60-228122    Patent Reference 9: Japanese Patent Application Laid-open No. S60-255415    Patent Reference 10: Japanese Patent Application Laid-open No. H6-136150
As described above, from the excellent mechanical properties thereof, the formed article of ultra-high molecular weight polyolefin has been studied for numerous diverse applications; however, nothing has been obtained that has an entirely novel function responding to new requirements from the industry and new requirements from the marked, which are changing vertiginously.
In particular, polyolefin, which is a crystalline resin, behaves very differently from other non-crystalline resins in terms of properties such as transparency and contractility. Consequently, if such a novel function could be achieved, specifically, if a formed article of ultra-high molecular weight polyolefin having excellent transparency and mechanical properties while being relatively thick could be realized, new applications are anticipated to spread.
However, owing to the height of the molecular weight thereof, a polymer of ultra-high molecular weight polyolefin entangles strongly, narrowing the broadness of material selection and the degree of freedom of the production process (process margin) in contrast to low molecular weight polyolefin, high density polyethylene and the like, such that a thick product provided with both functions of transparency and mechanical properties could not be realized.